Diamond Bar, a sprawling bedroom community in the San Gabriel Valley, became a city Tuesday when an incorporation measure was approved by an overwhelming 76.2% of those who voted. It was the third campaign in six years to incorporate the area. SIZE: 14.9 square miles POPULATION: Estimated between 44,930 and 75,901. DEVELOPMENT: It began after the 8,000-acre Diamond Bar Ranch was sold to developers for $10 million in 1956. REGISTERED VOTERS: 24,667 CITY COUNCIL (Elected Tuesday): Phyllis Papen, 44, a real estate agent and president of the Diamond Bar Improvement Assn. Paul Horcher, 32, an attorney and member of the Diamond Bar Municipal Advisory Council (MAC), an elected advisory board to Supervisor Pete Schabarum. Gary Werner, 45, a planning consultant and founder of the Incorporation ’88 Committee. Gary Miller, 40, owner of a development firm and founder of Concerned Citizens for Diamond Bar Traffic Control, a citizens group concerned with traffic resulting from the Grand Avenue extension. He was appointed to MAC by Schabarum in December. John Forbing, 45, insurance broker and president of the Walnut Valley Unified School District’s board. Papen and Horcher, because they were the top two vote-getters, will serve four-year terms. The others will serve for two years. The council will have its first official meeting some time in April. HOUSING: 14,191 total units, including 10,331 single-family homes and duplexes and 3,860 apartments and condominiums, according to the Regional Planning Department. SERVICES: The city would contract with the county for police and fire protection until the City Council creates city police and fire departments. (Council members say this will not happen any time soon.) MAJOR PROBLEMS: Growth and the resulting increase in traffic. Diamond Bar’s population, according to the regional planning staff, increased 60.2% from 1980 to 1988, making it the second-fastest-growing community in the San Gabriel Valley. Residents are upset by San Bernardino County’s extension of Grand Avenue to Chino Hills, which is scheduled to open later this month. Extension of the road, which now ends at the county line, would clog Grand Avenue in Diamond Bar with 38,100 cars a day--127% of the road’s designed capacity--by 1991, according to a study commissioned by San Bernardino County. Last month, Schabarum ordered a fence built at the Grand Avenue terminus to delay the extension’s opening until more roads are built in Chino Hills to divert traffic. It will be up to the city of Diamond Bar to wage a legal fight against San Bernardino County to keep the extension from opening.